Method and device for driving a gas discharge lamp

ABSTRACT

A method for dimming a gas discharge lamp ( 2 ), particularly a HID lamp, and more specifically a MH lamp, is described. The lamp is operated at nominal power with commutating DC current having a current magnitude I L =αI nom , α being equal to 1 or less than 1. The current magnitude I L  is reduced, but the lamp still is operated at commutating DC current, until a reaches a predetermined value β. Then, the lamp is operated at DC current, and the current magnitude is reduced further. It is now possible to achieve a lower dimming level.

The present invention relates in general to a method and a device fordriving a gas discharge lamp, specifically a HID lamp, more specificallya metal halide lamp. More particularly, the present invention relates todimming such a lamp.

Gas discharge lamps are commonly known. In general, they comprise alight transmitting vessel enclosing a discharge space in a gastightmanner, an ionizable filling and a pair of electrodes in the dischargespace, each electrode being connected to an associated current conductorwhich extends from the discharge space through the lamp vessel to theexterior. During operation, a voltage is applied across said electrodes,and a gas discharge occurs between said electrodes causing a lampcurrent to flow between the electrodes. Although it is possible to drivean individual lamp within a relatively wide range of operating voltagesand/or currents, a lamp is typically designed to be operated at aspecific lamp voltage and lamp current and thus to have a specificnominal power consumption. At this nominal power, the lamp will generatea nominal amount of light. Since HID lamps are commonly known to personsskilled in the art, it is not necessary to discuss their constructionand operation here in more detail.

Generally speaking, it is desirable for a lamp to be dimmable, i.e. thelamp can be operated at a power below the nominal power, such that thelamp will generate less light than the nominal light output. Forlow-pressure gas discharge lamps, it is for instance known to operatethe lamps with AC current and dim a lamp by applying the lamp voltageonly during a reduced phase of the lamp period, for instance by a properphase control of a triac switch in series with the lamp. This means thatthe lamp receives a lamp voltage only during part of the voltage period,while no lamp current flows during the remaining part of this voltageperiod. The required amount of dimming is obtained by selecting theratio between the current-on time and the current-off time. However,such type of dimming is not possible in HID lamps, because this type oflamp has problems recovering from a current-off period.

While a low-pressure gas discharge lamp is typically operated withresonant current, i.e. current having a sine-shaped waveform, ahigh-pressure discharge lamp is typically operated by supplyingcommutating DC current. An electronic ballast or driver for such a lamptypically comprises an input for receiving AC mains power, a rectifierfor rectifying the AC mains voltage to a rectified DC voltage, a DC/DCupconverter for converting the rectified mains DC voltage to a higher DCvoltage, a downconverter for converting said higher DC voltage to alower DC voltage (lamp voltage) and a higher DC current (lamp current),and a commutator for regularly changing the direction of this DCcurrent. The downconverter behaves like a controlled constant currentsource, also known as controlled constant current generator. Typically,the commutator operates at a frequency in the order of about 100 Hz.Therefore, in principle, the lamp is operated at a constant currentmagnitude, the lamp current regularly changing its direction within avery brief time (commutating periods). This mode of operation will beindicated as square-wave current operation.

In a HID lamp, dimming based on phase-cutting the lamp current leads to,for example, reignition problems. Therefore, this type of lamps can bedimmed more readily by decreasing the lamp current to a level below thenominal current. In practice, it is already known to dim HID lamps bydecreasing the lamp current to a value below the nominal current.

However, reducing the lamp current in a HID lamp causes problemstypically associated with HID lamps, and it is simply not possible toreduce the lamp current unlimitedly. In typical low-pressure fluorescentlamps, the lamp electrodes can be heated separately by electrodecurrent. However, this is not possible in HID lamps. In HID lamps, thelamp electrodes are heated by lamp current, and if the lamp current isreduced, the lamp electrodes cool down and do not function properlyanymore. This lamp behavior, more particularly this electrode behavior,results in a practical limitation of the dimming capabilities of a HIDlamp. If the dimming level is defined as the ratio between dimmedoperating power and nominal lamp power, it is difficult to achievereliable dimming levels of 50% or more, whereas a low-pressure gasdischarge lamp such as a commonly known fluorescent lamp can easily beoperated at a dimmed level of 10% or lower.

The above applies especially to metal halide lamps, which form a specialfamily within the generic type of HID lamps. In fact, some manufacturersdo not allow their lamps to be dimmed while others discourage it orprescribe a limit of 50% to the dimming level.

The present invention is based on a better understanding of the behaviorof HID lamps.

Under normal or nominal operating conditions, lamp electrodes operate ina so-called diffuse mode during their cathode phase. When current isreduced from nominal current to a lower current level, the lampelectrodes change to a so-called spot mode, involving a very hot localspot on the electrode during their cathode phase. When the current isdecreased still further, the lamp electrodes change to a glow mode andlamp operation changes to a glow discharge, which is undesirable forsteady-state operation.

A HID lamp is designed for optimal operation in the diffuse mode.Operation in the glow discharge mode is undesirable because sputteringoccurs, while the lamp generates little or no light. The spot mode wouldin principle be acceptable, but it appears that the spot cools down veryfast. In combination with current interruptions, this can lead to thelamp going out.

The present invention is based on the recognition that the spot mode isin fact relatively stable as long as it is not interrupted. Normally, asmentioned above, a HID lamp is operated with square-wave current, whichmeans that the lamp current is repeatedly changed in direction. Thismeans that, during a current period, an electrode is operated as acathode during 50% of the current period and as an anode during theother 50% of the current period. Thus, the spot-mode operation of anelectrode is interrupted when the current direction changes. It has beenfound that the lamp goes out because at the end of an anode period andat the beginning of a new cathode period, the electrode apparently isnot capable of returning into the spot mode. However, it has also beenfound that the spot mode is relatively stable as long as the cathodeoperation of the electrode continues.

Based on this recognition, the present invention proposes to switch toDC operation at reduced current levels.

Indeed, it has been found that, when a HID lamp is operated with DCcurrent, the current level can be reduced much further before the lampgoes out. This can be attributed to the stability of the spot mode,which apparently remains stable when the lamp current is lowered, aslong as one electrode is continuously being used as a cathode.

A further advantage resides in that the reduction in light output causedby aging can be decreased when a HID lamp is operated with dimmed DCcurrent.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which:

FIG. 1 is a graph illustrating lamp current as a function of time;

FIG. 2 is a diagram illustrating an exemplary embodiment of a drivingdevice for a lamp; and

FIGS. 3A-B are graphs illustrating lamp maintenance as a function oflamp life.

FIG. 1 is a graph illustrating the lamp current through a HID lamp as afunction of time, for different dimming levels.

At (a) the current is shown for nominal operation of the lamp. It can beseen that the current magnitude or absolute value of the lamp current isalways equal to I_(nom), but that the lamp current changes direction attimes t₁, t₂, t₃, etc., which is indicated as a change from +I_(nom) to−I_(nom) and vice versa. In this nominal mode of operation, the lamppower will be indicated as P_(nom).

At (b) a dimmed mode of operation is illustrated, where the lamp isstill supplied with square-wave current but the current magnitude orabsolute value I_(L) of the current is less than I_(nom), which isexpressed by the formula I_(L)=αI_(nom), where α<1. The lamp power inthis case is indicated as P(α), which is less than P_(nom). According tothe invention, a HID lamp is dimmed with such a square wave currenthaving a current magnitude I_(L) as long as I_(L)/I_(nom) is larger thana predetermined value β. A suitable value for β has been found to beapproximately 60%, although in practice this will depend on the lamptype.

At (c) the DC mode of operation of the lamp is illustrated. Again, themagnitude I_(L) of the lamp current can be expressed as αI_(nom), butnow α is less than the above-mentioned predetermined value β.

In an experiment, three different HID lamps were tested.

First Experiment

A first test concerned a lamp of type CDM-T 70W/830, manufactured byPhilips Corporation, which is a lamp having a nominal lamp currentI_(nom) of about 0.85 A and a nominal power of 70 W. The lamp was firstoperated with a square-wave current as described above and illustratedin FIG. 1 at (a) and (b). The magnitude of the current was reducedslowly, until the lamp went out. This was found to occur at a lamp powerof about 35 W, corresponding to a dimming level of 50%, α being about0.5 when the lamp went out.

By way of comparison, the lamp was operated in accordance with themethod of dimming according to the present invention. Initially, thelamp was operated as illustrated in FIG. 1 at (a), at nominal power withnominal current. Then, as illustrated in FIG. 1 at (b), the currentshape still being a square wave, the lamp current magnitude I_(L) wasreduced from I_(nom) to αI_(nom), a being less than 1, untilα=I_(L)/I_(nom) reached a predetermined value β, which was taken to be60% in this experiment. Then, the commutation of the current wasstopped, i.e. the current was changed to DC current, as illustrated inFIG. 1 at (c). Subsequently, the lamp current magnitude I_(L) wasreduced still further until the lamp went out. This was found to occurat a lamp power of about 20 W, corresponding to a dimming level of 30%of the nominal power, α being about 0.3 when the lamp went out.

Second Experiment

A second test concerned a lamp of type SDW-T 100W, manufactured byPhilips Corporation, which is a lamp having a nominal lamp currentI_(nom) of about 1.1 A and a nominal power of 100 W. The same experimentas described above was performed. When operated with a square-wavecurrent, the lamp went out at a lamp power of about 40 W, correspondingto a dimming level of 40% of nominal power, α being about 0.5 when thelamp went out.

When operated in accordance with the method of dimming according to thepresent invention, the lamp went out at a lamp power of about 10 W,corresponding to a dimming level of 10% of the nominal power, α beingabout 0.3 when the lamp went out.

Third Experiment

A third experiment concerned a lamp of type CDM-T 150W/830, manufacturedby Philips Corporation, which is a lamp having a nominal lamp currentI_(nom) of about 1.7 A and a nominal power of 150 W. The same experimentas described above was performed. When operated with a square-wavecurrent, the lamp went out at a lamp power of about 60 W, correspondingto a dimming level of 40% of the nominal power, α being about 0.4 whenthe lamp went out.

When operated in accordance with the method of dimming according to thepresent invention, the lamp went out at a lamp power of about 30 W,corresponding to a dimming level of 20% of the nominal power, a beingabout 0.2-0.3 when the lamp went out.

Thus, for all of these tested lamps, the minimum power level attainablehas been reduced substantially by switching from square wave current toDC current.

It is noted that, although the exact value of β for switching fromsquare-wave current to DC current is not critical, this value should notbe taken too high, because at current levels close to nominal current, aHID lamp should not be operated with DC current. As will be known to aperson skilled in the art, the anode temperature is much higher duringDC operation than during AC operation. During dimmed DC operation, theanode temperature should preferably not rise above the electrodetemperature at nominal AC operation in order to avoid potentiallydetrimental effects.

It is known that the light generating capabilities of a lamp, expressedas light output per unit power, decreases as the lamp ages; this effectcan be expressed as maintenance, i.e. how does a lamp maintain itsoriginal properties, by plotting the light generating capabilitiesversus the lamp life. Using the DC mode for dimming appears to also havean advantageous effect on the maintenance of a lamp, which isillustrated by FIGS. 3A-B. Here, maintenance is expressed as apercentage of the original light generating capabilities.

FIGS. 3A-B show the results of experiments conducted on lamps of typeMHC070. Curves (a) to (c) of FIG. 3A relate to lamps driven withcommutating current, whereas curves (d) to (h) of FIG. 3B relate tolamps driven with constant (non-commutating) current. All lamps weresubmitted to a cycle of 12 hours, which was repeated constantly.

Curve (a) relates to a cycle of 11 hours at nominal power, followed by 1hour OFF. After 8000 hours, maintenance has decreased to about 70%.

Curve (b) relates to a cycle of 15 minutes at nominal power, followed by10 hours 45 minutes burning at 60% of the nominal power, followed by 1hour OFF. After 8000 hours, maintenance has decreased to almost 50%; areduction to 70% is reached already after 2000 hours.

Curve (c) relates to a cycle of 5.5 hours at nominal power, followed by5.5 hours burning at 60% of the nominal power, followed by 1 hour OFF.After 4000 hours, the maintenance has decreased to almost 70%.

It can be seen that maintenance is reduced as a lamp ages, while dimmingcauses the extent of the reduction to increase.

Curve (d) relates to a cycle of 11 hours at nominal power, followed by 1hour OFF. After 8000 hours, maintenance has decreased to somewhat lessthan 80%.

Curve (e) relates to a cycle of 11 hours burning at 50% of the nominalpower, followed by 1 hour OFF. After 8000 hours, maintenance is stillabove 70%.

Curve (f) relates to a cycle of 11 hours burning at 30% of the nominalpower, followed by 1 hour OFF. After 4000 hours, the maintenance hasdecreased to somewhat less than 70%.

Curve (g) relates to a cycle of 5.5 hours at nominal power, followed by5.5 hours burning at 50% of the nominal power, followed by 1 hour OFF.After 8000 hours, the maintenance is still about 75%.

Curve (h) relates to a cycle of 5.5 hours at nominal power, followed by5.5 hours burning at 30% of the nominal power, followed by 1 hour OFF.After 4000 hours, the maintenance is still about 85%.

It follows that, even when a lamp is dimmed to a higher extent, thereduction in maintenance when using DC is less as compared to lampsburning on commutating current.

FIG. 2 schematically illustrates a possible embodiment of a driver 1 fordriving a HID lamp 2 in accordance with the invention. Since suchdrivers are generally known, a detailed description of the design andoperation of such drivers is not necessary here. A skilled person willrecognize that such a driver 1 has a controllable current generatingmeans 10, receiving an AC mains input voltage, and generating at anoutput 11 a DC current in response to a control signal S₁ received at acontrol input 12. This controllable current generating means 10 isfollowed by a commutator stage 20, which is shown in FIG. 2 in a fullbridge embodiment. Such commutator stage 20 typically comprises fourcontrollable switches 21A, 21B, 22A, 22B. A first pair of controllableswitches 21A, 22A is arranged in series, a node 23A between these twoswitches being connected to one lamp electrode. A second pair ofcontrollable switches 21B, 22B is likewise arranged in series, a node23B between these two switches being connected to the other lampelectrode. A switch driver 30 has four outputs 31A, 31B, 32A, 32Bconnected to respective control inputs of said switches 21A, 21B, 22A,22B. The switch driver 30 has two operative states. In a first operativestate, the output signals at its four outputs 31A, 31B, 32A, 32B aresuch as to open switches 21A and 22B while closing switches 21B and 22A,corresponding to a lamp current flowing through the lamp 2 in onedirection. In the other operative state, the output signals of theswitch driver 30 are such as to open switches 211B and 22A while closingswitches 21A and 22B, corresponding to lamp current flowing in theopposite direction. The switch driver has a control input 33; dependingon the value of a signal S_(C) received at its control input 33, theswitch driver 30 either alternates between the first operative state andthe second operative state (commutating mode) or the switch driver 30 isconstantly in one of those two operative states (non-commutating mode).In other words, the control signal S_(C) at the control input 33 of theswitch driver 30 controls whether the lamp current is commutating ornot. Hereinafter, this control signal S_(C) will be assumed to be adigital signal having two possible values CM (commutating mode) and NCM(non-commutating mode).

According to the present invention, such a driver 1 is provided with adim control unit 40 having one output 41 connected to the control input12 of the controllable current generating means 10 for controlling thecurrent level, and having a second output 42 for controlling theoperation of the commutator stage 20. This second output 42 is connectedto said control input 33 of the switch driver 30. The dim controller 40has a user input 43 for receiving a user command, thus allowing a userto set a desired dim level.

In response to the setting of its user input 43, the dim controller 40generates a corresponding control signal S₁ at its first output 41, forcontrolling the controllable current generating means 10 in order togenerate a corresponding current level. If the desired current level isabove a predetermined value β, the dim controller 40 generates, at itssecond output 42, an output signal S_(C) having a first value CM. Aslong as the output signal S_(C) at the second output 42 of the dimcontroller 40 has this first value CM, indicating a dim level between βand 1, the lamp current is commutating. If the desired current level isbelow said predetermined value β, the dim controller 40 generates, atits second output 42, an output signal S_(C) having a second value NCM.As long as the output signal S_(C) at the second output 42 of the dimcontroller 40 has this second value NCM, indicating a dim level below β,the lamp current has a constant direction.

Although the present invention has been explained in the foregoing bydescriptions of a few exemplary embodiments, it should be clear to aperson skilled in the art that the present invention is not limited tosuch embodiments; rather, various variations and modifications arepossible within the scope of protection of the invention as defined inthe appending claims.

For instance, it should be clear that inhibiting the commutationoperation of the switch driver in a standard type commutator can beachieved in many ways, the embodiment depicted in FIG. 2 onlyillustrating one of the many possibilities of achieving this.

Furthermore, although the embodiment of FIG. 2 is depicted as a modulardesign, it is also possible that the dim controller 40, and even switchdriver 30, are implemented as one integrated unit.

In the above, dimming has been described as decreasing the lamp currentfrom the nominal lamp current to a lower current level. However, it willbe clear to a person skilled in the art that, during dimmed operation,the dimming level can be increased as well as decreased. Increasing thedimming level involves increasing the lamp power and increasing the lampcurrent magnitude. So, the lamp current is increased as a DC current aslong as I_(L)/I_(nom)<β, and the lamp current is increased as analternating DC current as soon as I_(L)/I_(nom)>β.

1. Method for operating a gas discharge lamp, particularly a HID lamp,and more specifically a MH lamp, wherein: the lamp is provided withcommutating DC current at a current level I_(L)=αI_(nom) for β<α≦1, andwherein the lamp is provided with DC current at a current levelI_(L)=αI_(nom) for α≦β wherein I_(L) represents the actual lamp current;I_(nom) represents the nominal lamp current; and β is a predeterminedvalue less than
 1. 2. Method according to claim 1, wherein β isapproximately equal to 0.6.
 3. Method for dimming a gas discharge lamp,particularly a HID lamp, and more specifically a MH lamp, the methodcomprising the steps of: operating the lamp at nominal power withcommutating DC current having a current magnitude I_(L)=αI_(nom), αbeing equal to 1 or less than 1; reducing the current magnitude I_(L),but still operating the lamp at commutating DC current, until α reachesa predetermined value β; providing the lamp with DC current of currentmagnitude I_(L)=αI_(nom) when a has reached the value β; and furtherreducing the current magnitude, but still providing the lamp with DCcurrent.
 4. Method according to claim 3, wherein β is approximatelyequal to 0.6.
 5. Driver for a gas discharge lamp, designed to performthe method according to claim
 1. 6. Driver according to claim 5,comprising: controllable current generating means for generating asubstantially constant current; and controllable commutating meansdesigned to commutate said current if the current magnitude exceeds apredetermined current level and to output said current as a DC currentif the current magnitude is below this predetermined current level. 7.Driver according to claim 5, comprising a user-adjustable control unithaving a first control output for generating a control signalcontrolling the current magnitude of the current generator and a secondcontrol output for generating a control signal controlling thecommutating means, wherein the control unit is adapted to switch thecommutating means to a commutating mode if the controlled currentmagnitude is larger than said predetermined current level and to switchthe commutating means to a DC mode if the current magnitude is belowsaid predetermined current level.